The present invention relates to improvements in pressureless tennis balls, and more particularly to pressureless tennis balls having excellent properties and feel of striking comparable to those of pressurized tennis balls.
Tennis balls are classified into pressurized tennis balls and pressureless tennis balls. The pressurized tennis balls generally consist of an inner hollow core (core ball) made of a rubber or a rubber-like elastomer containing air or a gas at a pressure about 0.6 to 0.9 kg./cm..sup.2 higher than the atmospheric pressure and a textile or felt covering. The pressureless tennis balls generally consist of an inner hollow core containing air at atmospheric pressure and a textile or felt covering.
The pressurized tennis balls have the disadvantage that the gas or air of super-atmospheric pressure contained in the core gradually diffuses out through the core wall owing to a pressure difference between the inside and outside of the core and the internal pressure decreases in several months. Consequently, the rebound properties, namely the flight performance, of the ball is reduced, and the tennis balls are no longer satisfactorily used. It is accordingly necessary for the pressurized tennis balls to be used within a certain specified time after manufacture or to be kept in pressurized containers prior to use for preventing or decreasing the lowering of the internal pressure. However, such a care is inconvenient and expensive.
In order to eliminate these disadvantages, various pressureless tennis balls have been proposed. For instance, U.S. Pat. No. 2,896,949 discloses a pressureless tennis ball made from a core composition containing rubber and 10 to 45 parts by weight of a high styrene-butadiene copolymer per 90 to 55 parts by weight of rubber. Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 96171/1980 discloses a pressureless tennis ball having a core made from a rubber composition containing either a copolymer of ethylene and propylene or a terpolymer of ethylene, propylene and a non-conjugated diene monomer in an amount of at most 60% by weight based on the total weight of the whole polymers. Japanese Unexamined Patent Publication No. 34934/1979 discloses a pressureless tennis ball made from a core composition containing as a polymer component 10 to 30% by weight of an ionomer resin, 30 to 70% by weight of natural rubber and 50 to 80% by weight of cis-1,4-polybutadiene. It is also proposed to incorporate in a core composition for a pressureless tennis ball 20 to 50% by weight of wood flour as a reinforcing filler based on a rubber, as known from British Pat. No. 1,108,556.
However, any pressureless tennis balls available at the present time are not used in high class tennis tournaments, since the pressureless tennis balls do not give a satisfactory feeling at the time of striking by a racket or the softer ones are bad in flight and do not give a feeling of striking like pressurized tennis balls, and moreover the lowering of compression is marked owing to repeated forceful striking in the course of game or playing.
It is an object of the present invention to provide a pressureless tennis ball which has none of the drawbacks of conventional pressureless tennis balls and has characterisitcs comparable to those of pressurized tennis balls.
This and other objects of the present invention will become apparent from the description hereinafter.